Understanding compression ratios – fixed vs. variable

Variable compression provides both power and efficiency within one engine for greater flexibility

VC-Turbo can shift to any compression ratio between 8:1 and 14:1

The compression ratio is the ratio of the combustion chamber’s volume from its greatest capacity (when the piston is at bottom-dead-center within the cylinder) – to its smallest capacity (when the piston is at top-dead-center).

A higher compression ratio is able to achieve greater efficiency and improved fuel consumption; lower compression ratios offer greater power and torque, particularly in turbocharged engines, but are known for reduced fuel efficiency.

Until now, in every production engine ever built, this ratio has been fixed as a result of the dimensions, components and engine layout.

INFINITI’s VC-Turbo engine uses an advanced multi-link system to change the compression ratio seamlessly, actively raising or lowering the height of the pistons’ stroke. Offering any compression ratio between 8:1 and 14:1, the VC-Turbo engine offers both power and efficiency within the same engine.

As well as offering the benefits of true flexibility, VC-Turbo technology negates the downsides posed by fixed compression. INFINITI’s engine neatly sidesteps the problem of ‘knocking’, a process of inefficiency which can occur in higher compression ratio engines when the air-fuel mixture combusts prematurely in the cylinder, potentially resulting in damage to the engine. INFINITI’s VC-Turbo engine avoids this with direct injection fuel delivery, and by constantly optimizing the compression ratio. This latter characteristic ensures greater efficiency than many high performance engines with a fixed, low compression ratio.

Due to the advantages of the engine’s variable compression ratio, as well as the multitude of other innovations applied to the four-cylinder VC-Turbo engine, INFINITI engineers are targeting up to a 27% improvement in fuel efficiency over V6 engines of a similar power output, depending on its vehicle application.

In addition, gasoline VC-Turbo technology offers a dramatic reduction in emissions over diesel powertrains, such as particulate matter and NOx.

The VC-Turbo engine provides a power output of 200 kW (268 bhp, 272 ps), and 390 Nm (287 lb ft) torque. This impressive specific power output is higher than many four-cylinder turbocharged gasoline engines. In terms of performance, the 2.0-liter VC-Turbo engine will offer similar performance to some gasoline V6 powertrains, and with the potential for greater fuel efficiency.